DIFFERENTIAL CONTROL OF LUTEINIZING HORMONE AND FOLLICLE-STIMULATING HORMONE BY LUTEINIZING

HORMONE RELEASING HORMONE IN THE RAM G. A. LINCOLN MRC Unit of Reproductive Biology, 2 Forrest Road,

Edinburgh, EHI 2QW

(Received 3 May 1978) SUMMARY

Adult Soay rams with low concentrations of gonadotrophins in the circulation as a result of 12 weeks of exposure to long daylengths (16 h light : 8 h darkness) were given small doses (100 ng) of synthetic luteinizing hormone releasing hormone (LH-RH) into the jugular vein two, four or seven times/day for 10 days. Each injection of LH-RH induced a transitory increase in the concentration of LH and testosterone in the plasma, whereas the concentration of FSH showed little immediate change, After repeated treatment with pulses of LH-RH, the responses of LH and testosterone became slightly enhanced and the plasma concentration of FSH became permanently raised; these changes were most conspicuous in the animals receiving the most frequent

injections.

At the end of the study when the injections of LH-RH were stopped, the concentrations of LH and testosterone remained low but the concentrations of FSH continued to be maintained at a high level for at least 24 h. INTRODUCTION

Luteinizing hormone releasing hormone (LH-RH) appears to play a dual role in the ram, as in other mammalian species, in controlling the secretion of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. The evidence for this is that increases in the plasma concentrations of both gonadotrophins occur after the injection of LH-RH (Hopkinson, Pant